Colony development and reproductive success of bumblebees in an urban gradient

Abstract

Approximately 35% of all crop production is dependent on animal-mediated pollination. Many wild bee species are declining rapidly across North America and Europe, a potential consequence of land-use change driven by agricultural intensification and urbanization. In this study we assessed the impact of urbanization on the reproductive success and population growth rate of bumblebees in an urbanization gradient. We placed experimental nests in ten sites; all except one were community gardens, ranging from a 0-99% degree of urbanization. Reproductive success and colony size were positively correlated with cumulative weight gain of the nests (p < 0.05). We did not find an effect of urbanization on the population growth rate of the nests or on forager activity (p > 0.05). Growth rate was strongly negatively affected by the abundance of wax moth larvae (p < 0.05) and positively correlated with parasite diversity (p < 0.05) and the number of foragers entering the nest (p < 0.01). With this study we show that not only bottom-up but also top-down effects are equally important for pollinator population dynamics.